added option to specify keyring for serval sign
updated README.md
clean up indentations and added the option to select a keyring for verification
updated function definition for serval-verify
fixed keyring option functionality; lots of code cleanup
forgot some Makefile deps
Merge pull request #12 from opentechinstitute/issue6

Let serval-crypto to choose between keyring files (to keep identity lists separate). If no keyring file is specified, serval-crypto will use the default "serval.keyring" file. If the specified keyring does not exist, it will be created.

tested by areynold