Utility:tables_length() unlike # this can return the length of any table.

#table_name is only suitable for use with arrays of one contiguous part
without nil values:

http://stackoverflow.com/questions/2705793/how-to-get-number-of-entries-in-a-lua-table

This function is used by the audio code I've made for my WIP hell deaths
implementation.
Changed its name & made its for loop more readable
Merge pull request #172 from JASheppard/utility_tablesLength

Utility: Added new function table_length() that unlike '#' can return the length of any table