Massive update

* Added full math support
* Full support of predicate expressions
* Some new functions
* Some new axes + fixes for already (partially) implemented
* Unit tests
* Dialyzer + typespecs
* Readme
* Support for multiple arguments for functions like `concat(arg1, arg2, arg3, ..., argN)`
* Sure forgot smth else...

Nothing actually broken (backward-compatible)

See README for list of what is implemented and what not

XXX: not synced with master!
XXX: have 1 broken test that documents not-correct behaviour. But it may be successfully commented-out. https://github.com/retnuh/mochiweb_xpath/pull/5/files#L9R84
P.S.: I use this one in production and have plans to continue development (implement new functions/axes etc). So, new pull-requests are coming later.