Support to custom keystore, password, alias.

Currently Selendroid takes only keystore path for user defined
keystores, if the keystore set is not the android debug keystore
signing fails. This change allows user to specify custom keystore,
keystore password and alias.
Changes:
added two more switches -keystorePassword, -keystoreAlias, two set and
get methods in SelendroidConfiguration.
added a method in SelendroidBuilder to get Signature algorithm from the
keystore.
modified signTestServer to pass user defined password, alias in jar
signer command line.
added two tests to SelendroidServerBuilderTest, for signing test app
and server app to sing with custom keystore.

Tests run:
Units were successful.
Other tests with third party apps:
Tested with default keystore:
from command line with app installed and not installed,
with in the script with app installed and not installed.
Tested with custom keystore:
from command line with app installed and not installed
with in the script with app installed and not installed.