Add pwnlib.elf.ELF.entry and a few aliases for the entry point


```
>>> from pwn import *
>>> e = ELF('/bin/bash')
>>> print hex(e.address)
0x400000
>>> print hex(e.entry)
0x42020b
>>> print hex(e.symbols['_start'])
0x42020b
>>> context.arch = 'amd64'
>>> print e.disasm(e.entrypoint, 0x10)
  42020b:       31 ed                   xor    ebp,ebp
  42020d:       49 89 d1                mov    r9,rdx
  420210:       5e                      pop    rsi
  420211:       48 89 e2                mov    rdx,rsp
  420214:       48 83 e4 f0             and    rsp,0xfffffffffffffff0
  420218:       50                      push   rax
  420219:       54                      push   rsp
  42021a:       49                      rex.WB
```