Add gzip compression support
Looks like the avro-1.7 SchemaParser was also called during this path, and it cannot under the '@' chars in the field names (allowed in avro 1.4).

Use the RelaxedSchemaParser to get around this problem.

The stack trace when the error happened:

2014-11-01 09:14:52,842 WARN [Thread-5] org.apache.hadoop.mapred.YarnChild: Exception running child : org.apache.avro.SchemaParseException: Illegal initial character: @fields
        at org.apache.avro.Schema.validateName(Schema.java:1079)
        at org.apache.avro.Schema.access$200(Schema.java:79)
        at org.apache.avro.Schema$Field.<init>(Schema.java:372)
        at org.apache.avro.Schema.parse(Schema.java:1215)
        at org.apache.avro.Schema$Parser.parse(Schema.java:965)
        at org.apache.avro.Schema$Parser.parse(Schema.java:953)
        at org.apache.avro.Schema.parse(Schema.java:1005)
        at org.apache.avro.hadoop.io.AvroSerialization.getKeyWriterSchema(AvroSerialization.java:197)
        at org.apache.avro.mapreduce.AvroJob.getMapOutputKeySchema(AvroJob.java:174)
        at com.linkedin.camus.sweeper.mapreduce.AvroKeyMapper.setup(AvroKeyMapper.java:38)
Merge pull request #124 from HenryCaiHaiying/master

Looks like the avro-1.7 SchemaParser was also called during this path, a...
Merge pull request #122 from yagnik/support-gzip

Add gzip compression support