Update main.go

Added an option to allow for alpha-numeric passwords to be generated,
works just like the "all" option only minus the use of special 
characters.
Update main.go

Update usage printout.
Update README.markdown

Updated README to reflect addition of the 'alphanum' flag.
Update README.markdown

More grammatical changes to the README.markdown file, as well as
adding 'alphanum' argument to the examples list.
Update main.go

Fixed casing on "Case" as well as corrected spelling error on 
"numerals" for alpha numeric passwords.
Ran go fmt

Ran all source files and the test through 'go fmt'

Signed-off-by: Adam Jimerson <vendion@gmail.com>
Cleaned up the help text.
Also deleted some flag comments that repeated the help strings.