Deprecate rep(), head() and tail()

[nalimilan]

This package is not the place for utilities functions. Most cases
are supported by repeat(). rep(::AbstractVector, ::AbstractVector)
has no replacement currently, but it is a less common form and
should be moved to Julia Base or another package.

I'm trying to clean up DataFrames, and it extends this function. I figured we'd better prompt people to use Base's repeat ASAP. Though repeat does not support AbstractArrays until JuliaLang/julia#14082 is merged and backported, which is why I kept the existing functions instead of calling it.

I don't know what to do about rep(::AbstractVector, ::AbstractVector): should Base offer this feature via repeat? Or via another function? Should it go to a utilities package?

[coveralls]

Coverage decreased (-1.4%) to 82.693% when pulling 878aad7 on nl/rep into 44192cf on master.

[coveralls]

Coverage decreased (-1.4%) to 82.676% when pulling 1c48dee on nl/rep into 44192cf on master.

[nalimilan]

Any comments?

[coveralls]

Coverage decreased (-1.2%) to 70.244% when pulling 213a3ba on nl/rep into 3d53fbc on master.

[nalimilan]

I'll merge in a few days if nobody objects.

[johnmyleswhite]

+1 to this

[coveralls]

Coverage decreased (-0.6%) to 70.755% when pulling 3e00ab9 on nl/rep into 1b9f92f on master.

[coveralls]

Coverage decreased (-0.6%) to 70.755% when pulling 012b64b on nl/rep into 1b9f92f on master.