Cleaned up Subnet.java and added/extended unit tests.

Cleaned up Subnet.java and added/extended unit tests.

Subnet.java
  - Reordered modifiers to standard java lang spec order
  - Improved equals() with checks for null and this
  - Improved hashCode() by using all member vars used in equals().
  - Added braces to all if/else statements
  - Added constructor argument checks to ensure valid objects are created.
  - Renamed member variables to clearly identify what they are
  - Added javadoc
  - Added @Nullable and @Nonnull as appropriate

SubnetTest.java
  Fleshed out tests to cover all methods and cover more corner cases

PS2: minor javadoc error fix

Change-Id: I2b8097b096ee707ade064f6e583f72aba244e509
Signed-off-by: Ryan Vail <r.vail@cablelabs.com>