Dependency on seelog

[dolmen]

This library has a hard dependency on github.com/cihub/seelog. This is useless if we want to use death in a program that uses another logging package.

[vrecan]

you can replace the logger by calling SetLogger and passing it something that implements the logger interface

[dolmen]

The point of this issue is that the code of seelog will be embedded in the resulting executable even if I'm not using another logger.
This also implies that its huge list of dependencies will be embeded too:

$ go list -f '{{join .Deps "\n"}}' github.com/cihub/seelog
archive/tar
archive/zip
bufio
bytes
compress/flate
compress/gzip
container/list
context
crypto
crypto/aes
crypto/cipher
crypto/des
crypto/dsa
crypto/ecdsa
crypto/elliptic
crypto/hmac
crypto/internal/cipherhw
crypto/md5
crypto/rand
crypto/rc4
crypto/rsa
crypto/sha1
crypto/sha256
crypto/sha512
crypto/subtle
crypto/tls
crypto/x509
crypto/x509/pkix
encoding
encoding/asn1
encoding/base64
encoding/binary
encoding/hex
encoding/pem
encoding/xml
errors
fmt
github.com/cihub/seelog/archive
github.com/cihub/seelog/archive/gzip
github.com/cihub/seelog/archive/tar
github.com/cihub/seelog/archive/zip
hash
hash/crc32
internal/nettrace
internal/race
internal/singleflight
internal/syscall/unix
io
io/ioutil
math
math/big
math/rand
net
net/smtp
net/textproto
os
path
path/filepath
reflect
regexp
regexp/syntax
runtime
runtime/cgo
runtime/internal/atomic
runtime/internal/sys
sort
strconv
strings
sync
sync/atomic
syscall
time
unicode
unicode/utf8
unsafe
vendor/golang_org/x/crypto/chacha20poly1305
vendor/golang_org/x/crypto/chacha20poly1305/internal/chacha20
vendor/golang_org/x/crypto/curve25519
vendor/golang_org/x/crypto/poly1305
``` @