Add the hbase shell table_att for any attribute

Now the HTableDescriptor supports setValue(String key, String value) method, but the hbase shell not support it.

May be like this:

{quota}
hbase(main):003:0> alter 'test', METHOD=>'table_att', 'key1'=>'value1'
Updating all regions with the new schema...
1/1 regions updated.
Done.
0 row(s) in 1.0820 seconds

hbase(main):005:0> describe 'test'
DESCRIPTION ENABLED
{NAME => 'test', key1 => 'value1', FAMILIES => [{NAME => 'f1', BLOOMFILTER => 'NONE', RE true PLICATION_SCOPE => '0', VERSIONS => '3', COMPRESSION => 'NONE', MIN_VERSIONS => '0', TTL => '2147483647', BLOCKSIZE => '65536', IN_MEMORY => 'false', BLOCKCACHE => 'true'}]}
1 row(s) in 0.0300 seconds

hbase(main):007:0> alter 'test', METHOD=>'table_att_unset', NAME=>'key1'
Updating all regions with the new schema...
1/1 regions updated.
Done.
0 row(s) in 1.0860 seconds

hbase(main):008:0> describe 'test'
DESCRIPTION ENABLED
{NAME => 'test', FAMILIES => [{NAME => 'f1', BLOOMFILTER => 'NONE', REPLICATION_SCOPE => false '0', VERSIONS => '3', COMPRESSION => 'NONE', MIN_VERSIONS => '0', TTL => '2147483647', BLOCKSIZE => '65536', IN_MEMORY => 'false', BLOCKCACHE => 'true'}]}
1 row(s) in 0.0280 seconds{quota}