adds arguments parsing

adds main loop for parsing arguments; current supported arguments are:
-h -- Show help dialog.
-l -- List all available session files.

Implemented whith a while loop and parsed with the sh/bash builtin
'getopts' which gives the possibility of chainning arguments together
like so:
    `tmuxstart -hl`
or:
    `tmuxstart -h -l`
fixes -h option
fixes help menu when unknown options are passed
Merge pull request #11 from farfanoide/arguments

adds arguments parsing
adds basic completions for zsh and bash

updates readme with completions instructions

updates descriptions in completion files

updates completions instructions