Added fn.zip and fn.zipWith
Fixing some bad tests.

A new version of Lua or something similar seems to have caused some test bugs.
Now using improved tests that aren't order dependent.
Function renaming: zipWith -> zip_with
Merge branch 'master' into zip

Conflicts:
	fn/init.lua
Added tests of zip_with
Bugfix: zip of different-length tables
Merge pull request #2 from akfidjeland/zip

Added fn.zip and fn.zipWith
Fixing bug in repeat_val.
Added is_seq and flatten functions.
not treating tensors as sequences at the moment
checking whether a table has any data to determine whether it's a seq or map.
Adding thread function.
Adding take_nth.
Added identity function
Merge branch 'master' of https://github.com/rosejn/lua-fn
Added 'const' function
Fixing bugs in concat and mapcat where empty seqs resulted in premature ending of the output seq.
Fixing some tests and adding seq.group_by fn.