Check for invalid input

[fluvf]

Based on #46

• Add tests for invalid first argument (2fff651)
• Add test for null and undefined as children (1806e47)
• Check for invalid arguments (3983985 and 9b45acf)

With this Crel shouldn't crash anymore, if it's accidentally fed garbage

[fluvf]

Oh, I haven't look at this in a while... Just a sec, I'll get it rebased and take a look at the requests

[fluvf]

Rebased on #46 so I don't have to do it again anymore. Doesn't touch stuff touched by #47 either, so things should go smooth from here

[fluvf]

Now that I've taken another look at this, after a long break, I'm not too sure about how good of an idea this is. As it doesn't enforce legal tag names, Crel can still crash when createElement is called.
We're introducing some overhead over incomplete checks, that only catch corner cases, that wouldn't / shouldn't make it into production ready stuff anyways. I'm also not sure doing nothing is the best reaction, as it could hide bugs.

So should we do

• no checks at all (master)
• incomplete checks (this pr)
• or enforce correct tag names through regex

And if an invalid tag name is given, should crel:

• do nothing
• give an error message
• or just crash fast so it gets fixed fast

I'd also like to get your thoughts on what to do on empty crel() calls and should / could we introduce some default value to element? Another exposed property, that the user could set maybe?

(For the record, I do think 1806e47 and 9b45acf are good changes tho)