code reorganization (preparation for Arrow implementation)

[ExpandingMan]

I have reorganized the source code in preparation of rewriting Feather based on an underlying Arrow implementation.

Substantive Changes

• This includes my fix for method ambiguity when writing columns with eltype Union{DateTime, Missing} #70 (we can close fix writecolumn method ambiguity #71 and merge this instead).
• Added many type assertions in function definitions. For the most part, these are rather abstract, so don't have much of a tangible effect on the code. I find having lots of type assertions highly desirable because it screens out many possible errors, and (more importantly in this case) makes it much easier for people unfamiliar with the code to figure out what it's supposed to be doing.

Formatting Changes

• I have split the file "Feather.jl" into several distinct files. It is intended that these will completely change again once the Arrow implementation is complete. There is also a subdirectory called "arrow'. This is where Arrow development will occur for the time being until it is appropriate to split it off into a separate package.
• I have added several short convenience functions for commonly occurring operations.
• Various lines of code have been split into multiple lines for readability. I personally find very long run-on lines very difficult to parse. Also, the code is now easier for people who use editors with multiple columns to read.
• I added a tiny amount of (non user-facing) documentation.

Let me know if this seems acceptable.

[ExpandingMan]

Looks like tests are getting another method ambiguity error. Will look into this tomorrow.

[ExpandingMan]

I had accidentally left out some type assertions. Should be fine now.

Looks good on 0.6 on Linux. Mac build is hanging for some reason.

[ExpandingMan]

This may be sort of a moot point as Feather will pretty much have to be completely re-written with Arrow fully implemented. See #72 and my progress here.

[ExpandingMan]

This is no longer relevant. See my PR #78.