Implement -Z (--retrievable_by_cert) option.

Allow specified entity to retrieve credential w/o passphrase (trusted
retrivers)
Conflicts:

	myproxy/src/main/java/org/globus/myproxy/MyProxyCLI.java
Increase Myproxy default proxy key bits to 2048 (as in C version)
Fix myproxy -Z option
Conflicts:

	myproxy/src/main/java/org/globus/myproxy/MyProxyCLI.java
Merge pull request #56 from jrevillard/myproxy-Z-option

Add equivalent to myproxy-init -Z option (+ default proxy key bits increased to 2048 as in myproxy-init)