ACCUMULO-1933 Make unit on memory parameters case-insensitive.

1. Added support for both cases for G, M, K and B

2. Added warning message for lower case b and that the code will consider this bytes

3. Added meaningful error message for any number formatting issues

4. Added unit test that test 1 and 3 from above.

 modified:   src/core/src/main/java/org/apache/accumulo/core/conf/AccumuloConfiguration.java

 new file:   src/core/src/test/java/org/apache/accumulo/core/conf/AccumuloConfigurationTest.java