Fix compile error due to string literal overflow on large proto files.

[ken...@google.com]

Reviewers: eric.perie_hp.com,

Description:
Hi Eric, I decided to do this patch a little differently -- in
particular, I moved the actual string concatenation to occur in
Descriptors.java rather than in generated code. I also added a test.
At Google we have a policy of reviewing all changes before submission,
so can you take a look and tell me if this code looks good to you?

Please review this at http://codereview.appspot.com/99043

Affected files:
M java/pom.xml
M java/src/main/java/com/google/protobuf/Descriptors.java
M java/src/test/java/com/google/protobuf/DescriptorsTest.java
M src/Makefile.am
M src/google/protobuf/compiler/java/java_file.cc
A src/google/protobuf/unittest_enormous_descriptor.proto