Added unimplemented Blueprints method stubs

The new has(), hasNot(), and limit() methods are marked with warnings,
TODO comments and currently have no effect on the query.  I added
these just to get Titan compiling again while I work on Metrics.
Testing Metrics on Thrift Cassandra backend
Remove unchecked cast in AbstractElement#equals()

Modify AbstractElement's equals() method to eliminate potential
unchecked casts while attempting to stay as close as possible to the
behavior of the original implementation.

closes #286
Let AbstractVertex remove multivalued properties

This one-liner change brings AbstractVertex#removeProperty(String) and
(TitanKey) into conformance with the Javadoc contract in the
TitanElement interface.

This commit also adds a unit test that failed on the pre-commit
behavior and now passes on the post-commit behavior.

This commit only modifies and tests vertex property behavior.  I
haven't looked carefully at edge property method behavior
conformance to interface contracts.
Change default Astyanax backoff configuration

Issue #291 shows Astyanax's connection timeout logic generating a
negative sleep time.  The log timestamps in the issue suggest that the
Astyanax timeout configuration is the default.

By default, Titan uses Astyanax's BoundedExponentialBackoff to
generate timeouts.  Though Thread.sleep() eventually gets a long
value, BoundedExponentialBackoff and parent class ExponentialBackoff
use int arithmetic internally.  This leads to integer overflow and
wraparound to negative values when baseWaitTimeMS * (1 <<
(attemptcount + 1)) crosses Integer.MAX_VALUE.  Our default
configuration ran afoul of this on its final timeout attempt before
exhausting its 30 retries.

This commit changes the defaults to:
* base wait time: 100 ms
* maximum attempts/retries/exponent: 8
* maximum allowed wait time: 25000 ms

The maximum integer calculated in ExponentialBackoff will be 100 * (1
<< (7+1)) = 25600, well below overflow (and that will in turn be
capped at 25000).