Support for parallel test configuration

I've maven-surefire plugin configured with this options :
Concurrency config is parallel='classes', perCoreThreadCount=true, threadCount=4, useUnlimitedThreads=false  

This means, that maven runs 4 separate JVM, each JVM has own jacoco-agent and all agents needs to write to the same target/jacoco.exec file.

After all tests are finished, I've malformed target/jacoco.exec file.
```
Caused by: java.io.UTFDataFormatException: malformed input around byte 0
	at java.io.DataInputStream.readUTF(DataInputStream.java:639)
	at java.io.DataInputStream.readUTF(DataInputStream.java:547)
	at org.jacoco.core.data.ExecutionDataReader.readExecutionData(ExecutionDataReader.java:145)
```

Is there a synchronisation/write locking for target/jacoco.exec ?

I think that this is continuation of http://sourceforge.net/apps/trac/eclemma/ticket/191